The presently claimed invention relates to an Al.sub.2 O.sub.3 -coated cutting tool suitable for machining of metals by turning, milling, drilling or by similar chipforming machining methods.
Modern high productivity chipforming machining of metals requires reliable tools with excellent wear properties. This has so far been achieved by employing a cemented carbide tool body coated with a wear resistant coating. The cemented carbide tool body is generally in the shape of an indexable insert clamped in a tool holder.
The most commonly used wear resistant layers are TiC, TiN, and Al.sub.2 O.sub.3. Both single layer and multilayer coatings are employed. CVD, PVD or similar coating techniques are used for depositing the different layers onto the cemented carbide body.
During the past five to ten years, coated cemented carbide tools have been improved considerably with respect to reliability and tool life.
During, e.g., a turning and cutting operation, the coated tool is worn continuously on its rake face by the formed metal chip which causes crater wear. The machined workpiece also slides along the clearance face of the tool causing flank wear.
During high speed cutting, the tool edge may reach a very high temperature at the rake face. This leads to a diffusion type crater wear on the rake face of the tool. On the clearance face of the tool, the temperature is significantly lower mainly so that abrasive type wear occurs.
It is generally accepted that an Al.sub.2 O.sub.3 -layer performs best on the rake face due to its excellent ability to withstand diffusion type wear. Layers of the type MeC.sub.x N.sub.y O.sub.z, where Me is a metal selected from the group consisting of the Groups IVB, VB, and VIB of the Periodic Table, generally Ti and where (x+y+z)=1, which type is hereafter denoted by TiC.sub.x N.sub.y O.sub.z, generally performs better on the clearance face. Al.sub.2 O.sub.3 -layers on the other hand, wear relatively fast on the clearance face and develop flank wear relatively quickly on that face. The flank wear will be particularly large for thick, &gt;4 .mu.m, Al.sub.2 O.sub.3 -layers. Flank wear influences the machined surface and may therefore limit tool life. For TiC.sub.x N.sub.y O.sub.z -type layers, the situation is almost the reverse, that is, they exhibit low flank wear and faster crater wear than Al.sub.2 O.sub.3.
It is desirable to have a tool with high wear resistance on both the clearance face and on the rake face at the same time.
Other factors influencing cutting performance of a coated tool include spalling or flaking of the coatings. Flaking accelerates tool wear, in particular the flank wear. Flaking may be the result of inferior coating adhesion or it may be due to the smearing or welding of workpiece material onto the cutting edge and a successive withdrawal of the coating. This may occur when the adhesion strength between the chip formed and the coating material is sufficiently high.
Some steels are more difficult to machine than others due to smearing and resulting flaking, for example, stainless steel and low carbon steel.
Nowadays, less machining per each component is needed. The requirements for high surface finish of the machined component only allow tools with a clean smooth cutting edge-line with very little developed wear to be used. It is becoming more and more difficult for the machine operator by the naked eye to differentiate between a little used and an unused cutting edge ("edge identification"). This is particularly difficult if the top layer is Al.sub.2 O.sub.3 which color is dark grey or black. By mistake, using a used tool cutting edge, e.g., during an unmanned night shift run may cause component rejection or even unwanted production stops. Edge identification can more easily be done if the insert has a top layer of TiC.sub.x N.sub.y O.sub.z or in particular if the top layer is a goldish TiN-, ZrN- or HfN-layer.
In U.S. Pat. No. 4,643,620, the coating thickness is reduced along the edge by a mechanical treatment such as brushing, lapping or barrel polishing. The object is mainly to reduce the coating thickness along the cutting edge which is claimed to improve the toughness behavior of the cutting tool.
U.S. Pat. No. 4,966,501 discloses a method of reducing edge damages during cutting by reducing the coated surface roughness by employing a mechanical polishing, lapping or brush honing. This method is according to the findings of the present inventors not sufficient to minimize smearing.